Department of Environmental Systems Science, ETH Zurich, Zurich 8092, Switzerland.
Eawag Swiss Federal Institute of Aquatic Science and Technology, Dübendorf 8600, Switzerland.
Environ Sci Technol. 2022 Sep 20;56(18):13449-13460. doi: 10.1021/acs.est.2c03762. Epub 2022 Sep 2.
Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO, but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH) produces CO considerably more efficiently than aromatic carbonyls. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH group, regardless of other structural features. We then confirmed that all ArOCH-containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO and that lignin photodegradation may represent a source of hydroxyl radicals.
一氧化碳（CO）是溶解态有机质（DOM）光降解后第二丰富的产物，仅次于 CO，但形成机制尚不清楚。先前的工作表明，取代甲氧基的芳烃（ArOCH）的水相光降解比芳香族羰基化合物更有效地产生 CO。在此基础上，我们提出木质素的甲氧基芳基作为陆地 DOM 光化学形成 CO 的 C 源，通过两步途径：甲氧基的水解脱甲基生成甲醇和甲醇氧化生成 CO。为了验证该机制的合理性，我们研究了八种木质素模型化合物的光化学。我们首先观察到，初始 CO 生成速率与初始底物降解速率呈正相关，仅对于至少含有一个 ArOCH 基团的模型，而与其他结构特征无关。然后我们证实，所有含有 ArOCH 的底物都通过检测甲醇和相应的酚类转化产物经历了正式的水解脱甲基。最后，我们表明，所有模型在辐照过程中都会形成羟基自由基，这可能是引发甲醇氧化生成 CO 的氧化剂。这项工作提出了一个明确的机制，将普遍存在的、丰富的、易于量化的 DOM 功能与 CO 光生产联系起来。我们的结果进一步表明，甲醇可能是一种丰富（但被忽视）的 DOM 光产物，也是甲醛、甲酸和 CO 的可能前体，木质素的光降解可能代表了羟基自由基的来源。
